Welcome to the Dutch Endocrine Meeting 2024

Brown adipose tissue shows a day-night rhythm

Brown adipose tissue shows a day-night rhythm in the excretion of mitochondrial proteins in extracellular vesicles

Maaike van Buuren1, Melanie Modder1, Patrick Rensen1, Robin van Eenige1, Sander Kooijman1

(1) Division of Endocrinology, Department of Medicine and Einthoven Laboratory for Experimental Vascular Medicine, Leiden University Medical Center, Leiden, The Netherlands.

Aim:

Brown adipose tissue (BAT) combusts lipids to produce heat, and stimulating its activity has potential as treatment for atherosclerotic cardiovascular disease. Interestingly, BAT metabolic activity shows a strong day-night rhythm, coinciding with high uptake of triglyceride (TG)-derived fatty acids (FAs) from the circulation at wakening (i.e. Zeitgeber time 12; ZT12). To maintain its activity, thermogenically active BAT ejects proteins derived from damaged mitochondria in extracellular vesicles (EVs), which are phagocytosed by macrophages (Cell Metab 2022). Here, we studied whether excretion of mitochondrial proteins from BAT in EVs follows a similar day-night rhythm, and whether BAT-resident macrophages are critical for this rhythm.

Methods:

Male C57BL/6J mice were injected with clodronate liposomes to deplete BAT from macrophages, or empty liposomes as a control. Next, mice were injected with glycerol tri[3H]oleate-labeled triglyceride-rich lipoprotein-like particles at the onset of the light phase (ZT0) or dark phase (ZT12). BAT was collected to determine the uptake of [3H]oleate. From BAT, EVs were isolated and characterized using Western blot.

Results:

In line with previous findings, we observed a strong day-night rhythm in the uptake of TG-derived FAs by BAT (ZT12:ZT0=2:1). Release of the mitochondrial protein pyruvate dehydrogenase complex component X (PDHX) in EVs by BAT showed a rhythm (ZT12:ZT0=2.5:1) in phase with FA-uptake. Macrophage depletion decreased EV PDHX content at ZT12 (-72%), without affecting total EV excretion. These changes coincided with increased TG-derived FA uptake by BAT compared to controls (ZT12: +94%; ZT0: +208%), with loss of rhythmicity.

Conclusion:

Ejection of the mitochondrial protein PDHX from BAT in EVs shows a day-night rhythm in phase with BAT metabolic activity. Depletion of BAT-resident macrophages decreased PDHX excretion, and resulted in a loss of rhythmicity of TG-derived FA uptake. Surprisingly however, overall TG-derived FA uptake by BAT increased. Current experiments focus on understanding underlying mechanisms.